In PET imaging a radiotracer is administered to a subject such as a patient or an animal prior to its positioning in the PET imaging region. The radiotracer is preferentially absorbed by regions in the subject and its distribution is imaged following an uptake period. Subsequently a clinician interprets the relative uptake in particular regions in the images such as organs and may perform a diagnosis of the subject. The radiotracer undergoes radioactive decay which results in the production of positrons. Each decay event produces one positron which travels up to a few mm in human tissue where it subsequently interacts with an electron in an annihilation event that produces two oppositely-directed gamma photons. The two gamma photons each have an energy of 511 keV and are subsequently detected by gamma photon detectors disposed radially around the PET imaging region which each produce an electrical signal when struck by an incident gamma photon. In a gamma photon detector, defined herein to comprise a scintillator element in optical communication with an optical detector, the scintillator element converts the high energy gamma photon into a scintillation light pulse comprising a number of optical photons, and the electrical signal is generated by the optical detector. A timestamp is issued to each electrical signal by a timestamping unit and compared to other timestamps in a coincidence determination unit. Two gamma photons are identified as coincident if their timestamps occur within a narrow time interval of each other; typically if they are within +/−3 ns. The positions of the two detectors receiving the coincident gamma photons define a line in space along which the annihilation event occurred, the line being termed a line of response (LOR). Such LORs are subsequently reconstructed to produce an image illustrative of the distribution of the radiotracer within the imaging region. In time-of-flight (TOF) PET the small time difference between the two detected gamma photons is further used to localize the position along the LOR at which the annihilation event occurred, and thus improve the spatial resolution of the reconstructed image. In depth-of-interaction (DOI) PET the trajectories of the two detected gamma photons may further be assessed in order to improve the spatial resolution of the reconstructed image by reducing parallax errors.
In PET imaging systems in general the timestamping unit that issues timestamps to the received gamma photons typically includes a timing unit such as a time-to-digital converter (TDC), and a timestamp trigger unit. The timing unit is caused by the timestamp trigger unit to generate a timestamp indicative of the time of reception of each gamma photon for subsequent analysis by the coincidence determination unit. The timestamp trigger unit causes the timing unit to generate a timestamp when a signal at its input exceeds a predetermined threshold and desirably occurs as soon as possible after the detection of the gamma photon in order to optimize the timing accuracy of the PET imaging system.
A timestamping unit used for timing purposes in the direct detection of radiation quanta such as Cherenkov radiation operates in much the same way. In the detection of Cherenkov radiation however the optical detector generates the electrical signal directly from the detected radiation quanta, thus in the absence of a scintillator element.
False triggering of the timing unit is a problem that can arise in systems employing so-called direct detection, as well as in systems employing indirect detection such as PET imaging systems, and is particularly acute in such which employ digital silicon photomultiplier (SiPM) detectors operating in the Geiger mode as the optical detector. Digital SiPM detectors suffer from dark count noise which manifests itself as spurious electrical pulses at the output of the optical detector in the absence of a valid event such as an optical pulse or a received gamma photon. The electrical pulses from dark count noise are frequently misinterpreted by the timestamp trigger unit and falsely cause the timing unit to generate a timestamp. Such false triggering results in a timing unit deadtime, a period of time during which the timing unit must be reset and during which it is unable to determine the time of reception of valid events. Dark count noise is strongly temperature dependent, and even at room temperatures can create considerable timing unit deadtime.
Some discrimination between dark count noise and the signals from valid events can be achieved by raising the timestamp trigger unit's threshold. This however has limited benefits since the discredited nature of the electrical pulses resulting from both dark count noise and from valid events risks that some valid events do not create a sufficiently large signal to trigger the timestamp trigger unit. The missing of valid events degrades the detection sensitivity. In PET imaging it degrades the system's signal to noise ratio. Raising the timestamp trigger unit's threshold has a further drawback of increasing the time delay between the reception of a valid event and the time of its timestamp, thereby degrading the timing accuracy of the timestamping unit.
Two further methods have also been introduced to mitigate the effects of dark count noise: cooling the optical detector and trigger validation. Cooling the optical detector reduces the dark count noise through its temperature dependence and is typically a requirement of such imaging systems. It requires the attachment of bulky cooling apparatus to the optical detectors which adversely impacts system size, cost and power requirements. Trigger validation has also been employed in the context of PET imaging. In this, electrical signals from optical detectors responsive to scintillation light pulses resulting from gamma photons are used to generate a trigger validation signal indicative that the trigger signal originated from a gamma photon as opposed to dark count noise. In a known triggering scheme disclosed in patent application WO2006/111883A2 a timestamp trigger unit causes a timing unit to generate a timestamp when the first optical photon in the scintillation light pulse has been detected. The trigger validation scheme issues a corresponding validity signal based on a logical AND/OR of several such optical detector signals which is true when a predetermined number of such optical detector signals subsequently exceed the threshold. A valid trigger of the timing unit consequent to the reception of a gamma photon is characterized by the detection of its scintillation light at multiple optical detectors and causes a true validity signal, resulting in the processing of the timestamp. By contrast dark count noise triggers fewer optical detectors and causes a false validity signal, resulting in the timestamp being rejected and the timing unit being reset.
These solutions however still suffer from the drawback that the optical detectors must be cooled in order to reduce the dark count noise to an acceptable level. Furthermore the timestamp trigger unit's high threshold restricts the achievable timing resolution.